1. Field of the Invention
The present invention relates to hologram recording/reproducing apparatuses and light source apparatuses.
2. Description of the Related Art
Recently, a hologram memory has drawn attention as a data-storage device. In the hologram memory, a recording operation is performed using a hologram recording apparatus, and a reproducing operation is performed using a hologram reproducing apparatus. The recording operation is performed as follows: A signal light beam that has been modulated on the basis of data to be recorded and a predetermined reference light beam are generated using laser light emitted from the same light source; and The signal and reference light beams are applied to a hologram-recording medium, and are caused to interfere with each other in the hologram-recording medium to form interference fringes (a hologram). In this manner, data is recorded in the hologram-recording medium as the hologram. In the hologram that is recorded, an extremely large amount of information is included in units of one page. The recorded data is specified and managed in units of one page.
Additionally, in the hologram memory, the recorded data is reproduced from the hologram-recording medium, in which the data is recorded, using the hologram reproducing apparatus. The reproducing operation is performed as follows. A reproduction light beam having characteristics similar to those of the reference light beam that is used in the recording operation is applied to the hologram, which is formed on the basis of the recorded data, whereby a diffracted light beam is generated by the hologram-recording medium. Information expressed by the diffracted light beam corresponds to the recorded data included in one page. Accordingly, the diffracted light beam is received by photoreceptors that are two-dimensionally disposed, and signal processing is performed to reproduce the recorded data.
Furthermore, a hologram recording/reproducing apparatus has been suggested, in which a function of the hologram recording apparatus and a function of the hologram reproducing apparatus are realized in the same apparatus.
The generation of the signal, reference, reproduction light beams and the reception of the diffracted light beam, which are described above, are performed in an optical unit of the hologram recording/reproducing apparatus, and the optical unit is configured using a combination of optical components. As one system for designing an optical path in the optical unit, a so-called coaxial system (for example, see pages 106 to 114 of the Jan. 17, 2005 issue of Nikkei Electronics) is known, in which the signal and reference light beams are coaxially applied, in which the path of the reference light beam is the same as that of the reproduction light beam, and in which the light beams (the signal, reference, reproduction light beams) have a common optical path in which the light beams pass. As another system for designing an optical path in the optical unit, a two-beam system is known, in which the signal light beam and the reference (reproduction) light beam pass in different optical paths.
When the hologram is recorded and reproduced, a system is used, in which the light beam emitted from the single light source is split into the reference and signal light beams, and in which the interference fringes formed by the reference and signal light beams are recorded. Accordingly, as the light source, a coherent single-mode laser light source is necessary. As light sources including the above-described light source, some laser sources such as solid laser sources using yttrium aluminum garnet (YAG) crystals or the like or light sources using semiconductor lasers have been considered. From among the laser sources, external-cavity-type single-mode light sources (hereinafter, referred to as “external-cavity light sources”) using semiconductor lasers have been suggested as blue lasers (having a wavelength of 400 nm). As typical examples of the external-cavity light sources, external-cavity light sources of Littrow type and Littman type have been known. Japanese Unexamined Patent Application Publication No. 5-206580 discloses an external-cavity-type light source, and Japanese Unexamined Patent Application Publication No. 11-242424 discloses a system and method for a holographic storage device using a holographic optical element.